Typically, a switch mode power supply includes a main switching transistor coupled to a primary winding of a main power transformer. Output supply voltages are developed from voltages developed in secondary windings of the transformer. When the transistor is conductive, a current pulse is developed in the primary winding of the transformer and in the main switching transistor. A voltage is also developed in a current sensing resistor coupled in series with the main switching transistor. The voltage developed in the current sensing resistor is coupled to a comparator transistor. During a given conduction interval of the transistor, a latch that includes the comparator transistor is triggered when the current sensing resistor voltage exceeds a threshold voltage of the comparator transistor. An output of the latch is coupled to the control electrode of the transistor for controlling the turn off instant of the transistor on a current pulse-by-current pulse basis.
In one prior art zero voltage switching (ZVS) power supply, a resonant voltage pulse is produced when the transistor is turned off. The latch maintains the transistor turned off, during a portion of the resonant voltage pulse that follows the instant when the main switching transistor turns off. At the end of a trailing edge of the resonant voltage pulse, when the voltage across the transistor is at or close to zero volts, the transistor is turned on again to provide ZVS power supply. It may be desirable to sustain the main switching transistor in the turned off state, during the aforementioned portion of the resonant voltage, without using a latch for simplifying the circuitry.
A switch mode power supply, embodying an inventive feature includes a source of an input supply voltage and a supply inductance coupled to the input supply voltage source. A capacitance is coupled to the supply inductance to form a tuned, resonstant circuit. A first switching transistor is responsive to a first control signal for generating in a given peroid a pulse in the suppy inductance that is coupled to a load and for generating a resonant pulse in the capacitance. The first control signal turns on the first switching transistor at an end of the resonant pulse for providing zero voltage switching and to turn off the first switching transistor in the given period. The first control signal is generated in accordance with a capacitive current in the capacitance, during the resonant pulse, for maintaining the first switching transistor turned off during a portion of the resonant pulse.